Liquid crystal mixture

ABSTRACT

Liquid crystal mixture comprising compounds of formula (I)
 
wherein 
                 
     R 1 : is H or a linear or branched alkyl group or branched alkenyl group   R 2 : stands for   a) H or F   b) a linear or branched alkyl group or a linear or branched alkenyl group   c) a radical 
                 
   wherein R 3 , R 4 , R 5 , R 6 : are an alkyl group   M 1 , M 2 : represent independently from another a single bond, —OC(═O), —C(═O)O—, —OCH 2 —, —NH—   A: is   a) a linear or branched alkan-α,ω-diylgroup or alkene-α,ω-diyl group   b) the group —C(═Y)— wherein Y is CH-Z with Z being phenylen-1,4-diyl, optionally substituted by one to three halogenatoms, alkyl or alkyloxy groups,   c) the group —CHY wherein Y is CH 2 -Z with Z being phenylen-1,4-diyl, optionally substituted by one to three halogenatoms, alkyl or alkyloxy groups   d) a group 
                 
 
wherein
   p, q are 0, 1 or 2;   M 3  is a single bond or —OC(═O)—, —C(═O)O—, —OCH 2 —, —CH 2 O—, —C≡C—, —CH 2 CH 2 —, —CH 2 CH 2 CH 2 CH 2 —   the radicals 
                 
 
are phenylen-1,4-diyl,
   X is H, OH, a linear or branched alkyl or alkyloxy group,   m: is 0 or 1   X and M 1 -(A) m -M 2 -R 2  together can constitute   a) a ring of 4 to 16 members,   b) a combination of two either directly linked or spiro-linked rings.

The present invention relates to a novel liquid crystal mixture. Moreparticularly, it relates to a chiral smectic or ferroelectric liquidcrystal mixture, which is chemically very stable against heat and light,thus being especially suitable for active matrix panels. A furtheraspect of this invention is the use of this liquid crystal mixture indisplays, especially active matrix displays. Yet another aspect of thisinvention are active matrix displays comprising such a mixture.

Since Clark and Lagerwall found Surface Stabilized Ferroelectric LiquidCrystals (SSFLC) in 1980 [N. A. Clark and S. T. Lagerwall, Appl. Phys.Lett., 36, 899 (1980)], these liquid crystals have attracted attentionsas display materials in the coming generation and a number of studieshave been carried out thereon.

There are two major advantages of FLC displays: the ‘pixel speed’ iseasily 100 or even 1000 times faster than nematics and secondly, due tothe bistability, the resolution of such displays is not limited by thematerial.

However, passive matrix FLC displays are limited by certain trade-offrelationships: the higher the number of scanning lines, the higher isthe frame addressing time and thus the speed of a display is always incompetition with its resolution.

With respect to the trade-offs of passive matrix FLC displays severalauthors have suggested to combine the active matrix with FLC. The firstapproach was made by Hartmann (IEEE Trans. Electron. Devices 1989,36(9), part 1, pp. 1895-9) in the 80s utilising the charge-controlledbistability of a quasi-bookshelf FLC with MOS-FET technology. The largePs values, however, prohibited higher resolution with a-Si TFT driving.

Takatoh et. al. (6^(th) International Conference on Ferroelectric LiquidCrystals, 1997, 20-24 Jul., Brest, France; M. Takatoh et al. 1998, SIDDigest, 1171-1174) have demonstrated an AM display based upon chiralsmectics using a very high P_(S) material driven with an active matrixwith polycrystalline Silicon-TFT. Nito et. al. (Nito et al., 1993,Journal of the SID, 1/2, 163-169.) have suggested a monostable AM-FLCwith much lower P_(S), however, with the disadvantage of a stripey FLCtexture which is not suitable for high contrast displays without furtherimprovements. Furue et. al. (Furue, H. et al., 1998, IDW '98, 209-212)suggested a polymer stabilized SSFLCD with a FELIX® mixture with amaterial having a moderate P_(S) value.

High P_(S)-values require long charge-up times on each of the pixels andare therefore incompatible with high resolution, i.e. a large number ofscanning lines. This is the reason why Takatoh et. al. have used aspecial poly-crystalline silicon active matrix that allows for highercurrent densities as compared to the standard and cost-effectiveamorphous silicon TFT.

Asao et. al. have presented a monostable FLC mode (Y. Asao et al., ILCC2000, Sendai, and Jpn. J. Appl. Phys. 38, L534-L536, 1999 therein called“half-V-shape FLC” mode; see also T. Nonaka et. al., Liquid Crystals26(11), 1599-1602, 1999, therein called “CDR” mode). Such displaysprovide, by virtue of their smaller Ps values, solutions for the grayscale problem and the resolution limitation caused by too large P_(S)values in active matrix panels.

A remaining problem in the application of TFT-LCD (e.g. monostable FLC)is the limited “holding ratio”, caused by the activity of the manifoldof charge carriers that are present in the pixel volume and which tendto dis-charge the pixel within too short a time. In particular for thefast switching displays with a very thin cell gap the problem of ioniccharges which build up depolarisation fields leads to significantlimitations (cp. Sasaki, Japan Display 1986, 62; Nakazono, Int. Dev.Res. Cent. Techn. Rep. IDRC 1997, 65; Naemura, SID Dig. Techn. Pap.1989, 242; Fukuoka, AM LCD 1994, 216; Takatori, AM-LCD 97 DIGEST. 1997,53; Takatoh, Polym. Adv, Technol. 11, 413 (2000)).

Thus it is an important factor to maintain a very low ion content and toachieve chemical stability against heat and light, both of which couldcause additional ion formation. This demand has in practice lead to theexclusion of any material comprising hetero atoms such as N, S, even Oin liquid crystal mixtures for active matrix (i.e. TFT or MIM)applications {cp. e.g. Petrov et al., Liq. Cryst. 19(6), 729 (1995) [CAN124:101494]; Petrov, Proc. SPIE-Int. Soc. Opt. Eng.(1995), 2408 [CAN123:241500]; Dabrowski, Biul. Wojsk. Akad. Techn. 48(4), 5 (1999) [CAN131:163227]; Kirsch, Angew. Chem., Int. Ed. 39(23), 4216 (2000) andreferences cited in these papers}. Whereas, so far, this couldsuccessfully be done for nematics (cp. e.g.

DE-A 1 962 9812, p. 12 to 16), there is hardly any smectic (S_(c))material without such hetero atoms (cp. Demus et al., Flüssige Kristallein Tabellen, vol. 1 and 2). Thus the use of fast switching smectics forTFT application is strongly limited if not prohibited and the range ofpotentially available nematics is strongly reduced.

The object of the present invention was therefore to provide a liquidcrystal mixture with very high resistivity or holding ratio irrespectiveof the comprisment of hetero atoms, especially a chiral smectic, inparticular ferroelectric or antiferroelectric liquid crystal mixtureswhich are suitable for active matrix panels, especially with respect towithstanding heat- or light-induced chemical stress while maintainingthe performance of a low-ion-content mixture.

The present invention provides a liquid crystal mixture, especially achiral smectic mixture, in particular a ferroelectric orantiferroelectric liquid crystal mixture, most particularly a monostableferroelectric liquid crystal mixture comprising one or several compoundsof formula (I)

wherein

-   R¹: is H or a linear or branched alkyl group of 1 to 20 C atoms or    linear or branched alkenyl group of 2 to 20 C atoms, wherein in    either case one —CH₂— group can be replaced by    cyclohexylen-1,4-diyl, or one or two —CH₂— groups can be replaced by    —O—, if non-adjacent to N, or by —C(═O)— or —Si(CH₃)₂—, and one or    more H of the alkyl or alkenyl group optionally being replaced by F    or CH₃.-   R²: stands for-   a) H or F-   b) a linear or branched alkyl group of 1 to 20 C atoms or linear or    branched alkenyl group of 2 to 20 C atoms, wherein in either case    one or two —CH₂— groups can be replaced by —O—, —C(═O)O—,    —Si(CH₃)₂—, it also being possible to replace one or more H of the    alkyl or alkenyl group by F or CH₃,-   c) for a radical    wherein independently from the respective meanings in (I)-   R³, R⁴, R⁵, R⁶: represent independently from another an alkyl group    of 1 to 8 C atoms-   M¹, M² stand: independently from another for a single bond, —OC(═O),    —C(═O)O—, —OCH₂—, or —NH—-   A: is-   a) a linear or branched alkan-α,ω-diylgroup of 1 to 20 or    alkene-α,ω-diyl group of 2 to 20 C atoms, it also being possible to    replace, if non-adjacent to M^(1/2), one or two non-adjacent —CH₂—    groups by —O—-   b) the group —C(═Y)— wherein Y is CH-Z with Z being    phenylen-1,4-diyl, optionally substituted by 1 to 3 halogen atoms,    alkyl or alkyloxy groups of 1 to 4 C atoms, with the proviso that M¹    and M² are —C(═O)O— and —OC(═O)—, respectively;-   c) the group —CHY wherein Y is CH₂-Z with Z being phenylen-1,4-diyl,    optionally substituted by 1 to 3 halogen atoms, alkyl or alkyloxy    groups of 1 to 4 C atoms, with the proviso that M¹ and M² are    —C(═O)O— and —OC(═O)—, respectively-   d) a group    wherein-   p, q are 0, 1 or 2, and the sum of p+q being ≧1-   M³ is a single bond, —OC(═O)—, —C(═O)O—, —OCH₂—, —CH₂O—, —C≡C—,    —CH₂CH₂—, —CH₂CH₂CH₂CH₂—-   the radicals    are independently from another phenylen-1,4-diyl, which optionally    can be substitued by one, two or three F, or cyclohexan-1,4-diyl,    which can optionally be substituted by one CN, CH₃ or F, or    pyrimidin-2,5-diyl, optionally substituted by one F,    pyridine-2,5diyl, which can optionally be substituted by one F, or    naphthalene-2,6-diyl, which can optionally be substituted by one,    two or three F, or 1,2,3,4-tetrahydronaphthalene-2,6-diyl (the    aromatic ring optionally substituted by one, two or three F), or    decahydronaphthalene-2,6diyl, or indane-2,5(6)-diyl, or    fluorene-2,-7-diyl, or phenanthrene-2,7-diyl, or    9,10-dihydrophenanthrene-2,7-diyl, or (1,3,4)thiadiazol-2,5-diyl, or    (1,3)thiazol-2,5-diyl, or (1,3)thiazol-2,4-diyl, or    thiophen-2,4-diyl, or thiophen-2,5-diyl, or (1,3)dioxan-2,5-diyl, or    piperidin-1,4-diyl, or piperazin-1,4-diyl-   X is H, OH, or a linear or branched alkyl or alkyloxy group of 1 to    20 C atoms, wherein one or two —CH₂— groups can be replaced by —O—,    —C(═O)O—, —Si(CH₃)₂—, it also being possible to replace one or more    H by F or CH₃-   m: is 0 or 1-   X and M¹-(A)_(m)-M²-R² together can constitute-   a) a ring of 4 to 16 members, optionally substituted by alkyl of 1    to 15 C atoms-   b) a combination of two either directly linked or spiro-linked rings    of independently from another 4 to 16 members, optionally    substituted by alkyl of 1 to 15 C atoms in either of the three    variants the rings are independently from another carbocycles or    carbocycles comprising B, N, O or S heteroatoms.

Preferably the mixtures comprise 0.01 wt.-% to 10 wt.-% of one orseveral compounds of formula (I).

Especially prefered are mixtures comprise 0.1 wt.-% to 5 wt.-% of one orseveral compounds of formula (I).

Preferably the mixtures comprise one or several compounds of thefollowing formulae

wherein

-   X is H-   R¹ is H or alkyl of 1 to 4 C atoms-   R³, R⁴, R⁵, R⁶ are CH₃-   and/or    wherein-   X is H-   R¹ is H or alkyl of 1 to 4 C atoms-   R³, R⁴, R⁵, R⁶ are CH₃-   and/or    wherein-   X is H-   R¹ is H or alkyl of 1 to 4 C atoms-   R³, R⁴, R⁵, R⁶ are CH₃-   and/or    wherein-   X is H-   R¹ is H or alkyl of 1 to 4 C atoms-   R³, R⁴, R⁵, R⁶ are CH₃.

Particularly prefered are mixtures comprising

wherein

-   X is H-   R¹ is H or alkyl of 1 to 4 C atoms-   R³, R⁴, R⁵, R⁶ are CH₃-   M¹ is —OC(═O)—-   M² is —C(═O)O—-   n is 4 to 12-   and/or    wherein-   X is H-   R¹ is H or alkyl of 1 to 4 C atoms-   R³, R⁴, R⁵, R⁶ are CH₃-   R² is H-   M¹ is —OC(═O)—-   M² is a single bond-   n is 8 to 20-   and/or    wherein-   X is H-   R¹ is H or alkyl of 1 to 4 C atoms-   R³, R⁴, R⁵, R⁶ are CH₃-   R² is H or an alkyl or alkyloxy group of 1 to 16 C atoms, wherein    one or two —CH₂— groups can be replaced by —O—, —OC(═O)—,    —Si(CH₃)₂—, it also being possible to replace one or more H by F or    CH₃-   M¹ is —OC(═O)—-   M² is a single bond    is phenylen-1,4-diyl, optionally substituted by one or two F, or    cyclohexylen-1,4-diyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl    and/or    wherein-   X is H-   R¹ is H or alkyl of 1 to 4 C atoms-   R³, R⁴, R⁵, R⁶ are CH₃-   R² is H or an alkyl or alkyloxy group of 1 to 16 C atoms, wherein    one or two —CH₂— groups can be replaced by —O—, —OC(═O)—,    —Si(CH₃)₂—, it also being possible to replace one or more H by F or    CH₃-   M¹ is —OC(═O)—-   M² is a single bond-   M³ is a single bond, —OC(═O)—, —OCH₂—    is phenylen-1,4-diyl, optionally substituted by one or two F, or    cyclohexylen-1,4-diyl, pyrimidine-2,5-diyl, or pyridine-2,5-diyl    is phenylen-1,4-diyl, optionally substituted by one or two F, or    cyclohexylen-1,4-diyl, pyrimidine-2,5-diyl, or pyridine-2,5-diyl    and/or    wherein-   X is H-   R¹ is H or alkyl of 1 to 4 C atoms-   R³, R⁴, R⁵, R⁶ are CH₃-   R² is H or an alkyl or alkyloxy group of 1 to 16 C atoms, wherein    one or two —CH₂— groups can be replaced by —O—, —OC(═O)—,    —Si(CH₃)₂—, it also being possible to replace one or more H by F or    CH₃-   M¹ is a single bond-   M² is a single bond    is phenylen-1,4-diyl, optionally substituted by one or two F, or    cyclohexylen-1,4-diyl,    and/or    wherein-   X is H-   R¹ is H or alkyl of 1 to 4 C atoms-   R³, R⁴, R⁵, R⁶ are CH₃-   R² is H or an alkyl or alkyloxy group of 1 to 16 C atoms, wherein    one or two —CH₂— groups can be replaced by —O—, —OC(═O)—,    —Si(CH₃)₂—, it also being possible to replace one or more H by F or    CH₃-   M¹ is a single bond-   M² is a single bond-   M³ is a single bond    is phenylen-1,4-diyl, optionally substituted by one or two F, or    cyclohexylen-1,4-diyl,

Several compounds of the formula (I) are commercially available. Thesynthesis of the not commercially available compounds of formulae (I)and (II), resp., or new D compounds of formulae (I), (II), (III) and(IV), resp., are performed in analogy to pertinent papers, e.g.Dagonneau et al., Synthesis 1984, pp. 895-916 [CAN 103:37294]; Rozantsevet al., ACS Symp. Ser. (1985), 280 (Polym. Stab. Degrad.), pp. 11-35[CAN 103:142668].

In particular the commercially available2,2,6,6-tetramethyl-piperidine-4-one and4-hydroxy-2,2,6,6-tetramethylpiperidine can serve as precursors for thenew compounds of formulae (I), (II), (Ill) and (IV).

E.g. (Ia), wherein M¹ and M² are single bonds, R² an alkyl group of 1 to20 C atoms, can be obtained by reacting 2,2,6,6-tetramethyl-4-piperidonewith an alkyl magnesium halide to render4-alkyl-4-hydroxy-2,2,6,6-tetramethyl-piperidine (in analogy toSkowronski et al., Pol. J. Chem. 54, 195, 1980)), dehydration to3,4-dehydro-4-alkyl-2,2,6,6-tetramethylpiperidine [corresponds toformula (IV)] and consecutive hydrogenation (e.g. in analogy to DE-A2258086). Alternatively Wittig reagents can be applied for the firststep (e.g. in analogy to Collum et al., J. Am. Chem. Soc. 113, 9575(1991)).

E.g. (Ia), wherein M¹ is —OC(═O)—, can be obtained by subjecting4-hydroxy-2,2,6,6-tetramethylpiperidine to esterification (in analogy toU.S. Pat. No. 4,038,280, Example 14) with an appropriate carboxylic acidderivative X—C(═O)(CH₂)-M²-R² (X: Cl, Br or OH).

E.g. (Ia1), wherein M¹ is —OC(═O)— and M² is —C(═O)O—, can be obtainedby subjecting 4-hydroxy-2,2,6,6-tetramethylpiperidine to esterification(in analogy to U.S. Pat. No. 4,038,280, Example 14) with an appropriatecarboxylic acid derivative X—C(═O)(CH₂)C(═O)X (X: Cl, Br or OH).

E.g. (Ib), wherein M¹ is a single bond, M² is —OCH₂—, R² an alkyl groupof 1 to 20 C atoms and

is phenylen-1,4-diyl, can be obtained by bringing into reaction2,2,6,6-tetramethyl-piperidine-4-one and phenol (in analogy to SU 631516; CA 90:54839) to render3,4-dehydro-4-(4-hydroxyphenyl)-2,2,6,6-tetramethyl-piperidine, which ishydrogenated to 4-(4-hydroxyphenyl)-2,2,6,6-tetramethyl-piperidine (inanalogy to DE-A-2 258 086). This material can be (in analogy to U.S.Pat. No. 4,038,280, Example 25) transfered to the above mentionedexamples of (Ib) by reacting with an appropriate alkyl halide X—R² (X:Halide, Tosylate, Mesylate; R² not H or F).

E.g. (Ib), wherein M¹ is a single bond, M² is —OC(═O)—, R² an alkylgroup of 1 to 20 C atoms and

is phenylen-1,4-diyl, can be obtained from above mentioned4-(4-hydroxyphenyl)-2,2,6,6-tetramethyl-piperidine by esterificationwith an appropriate carboxylic acid derivative X—C(═O)—R² (X: Cl, Br,OH; R² not H or F) (e.g. in analogy to U.S. Pat. No. 4,038,280, Example14).

E.g. (Ib), wherein M¹ is a single bond, M² is —OC(═O)— or —OCH₂—, R² analkyl group of 1 to 20 C atoms and

is cyclohexyl-1,4-diyl, can be obtained from above mentioned4-(4-hydroxyphenyl)-2,2,6,6-tetramethyl-piperidine by hydrogenation (inanalogy to DE-A-2 415 818) to4-(4-hydroxycyclohexyl)-2,2,6,6-tetramethyl-piperidine and consecutiveetherification with an appropriate alkyl halide X—R² (X: Halide,Tosylate, Mesylate; R² not H or F) or esterification with an appropriatecarboxylic acid derivative X—C(═O)—R² (X: Cl, Br, OH; R² not H or F)(e.g. in analogy to U.S. Pat. No. 4,038,280, Example 14 and 25), resp.

E.g. (Ic), wherein M¹ is a single bond, M³ is —OC(═O)—, R²H, F or analkyl group of 1 to 20 C atoms,

is phenylen-1,4-diyl or cyclohexyl-1,4-diyl and

is phenylen-1,4-diyl, optionally substituted by one, two or three F, orcyclohexyl-1,4-diyl, optionally substituted by one CN, CH₃ or F, orpyrimidin-2,5-diyl, pyridin-2,5-diyl, naphthalin-2,6-diyl, optionallysubstituted by one, two or three F, thiophen-2,4-diyl,thiophen-2,5-diyl, can be obtained by subjecting above mentioned4-(4-hydroxy-phenyl)-2,2,6,6-tetramethyl-piperidine and4-(4-hydroxycyclohexyl)-2,2,6,6-tetramethyl-piperidine, respectively, toesterification with a moiety (X: F, Cl, Br or OH) (e.g. in analogy toU.S. Pat. No. 4,038,280, Example 14).

E.g. (Ic), wherein M¹ is a single bond, M³ is —OCH₂—,

is phenylen-1,4-diyl or cyclohexyl-1,4-diyl and

is phenylen-1,4-diyl, optionally substituted by one, two or three F, orcyclohexyl-1,4-diyl, optionally substituted by one CN, CH₃ or F, orpyrimidin-2,5-diyl, pyridin-2,5-diyl, naphthalin-2,6-diyl, optionallysubstituted by one, two or three F, or thiophen-2,4-diyl,thiophen-2,5-diyl, can be obtained by subjecting (in anlogy to U.S. Pat.No. 4,038,280, Example 25) above mentioned4-(4-hydroxyphenyl)-2,2,6,6-tetramethyl-piperidine and4-(4-hydroxycyclohexyl)-2,2,6,6-tetramethyl-piperidine, respectively, toetherification with a moiety (X: Cl, Br, Tosylate, Mesylate)

E.g. (Ic), wherein M¹ is a single bond, M³ is a single bond, R²H, F oran alkyl group of 1 to 20 C atoms,

is phenylen-1,4-diyl or cyclohexyl-1,4-diyl and

is phenylen-1,4-diyl, optionally substituted by one, two or three F, orcyclohexyl-1,4-diyl, optionally substituted by one CN, CH₃ or F, orpyrimidin-2,5-diyl, pyridin-2,5-diyl, naphthalin-2,6-diyl, optionallysubstituted by one, two or three F, thiophen-2,4-diyl,thiophen-2,5-diyl, can be obtained by subjecting above mentioned4-(4-hydroxy-phenyl)-2,2,6,6-tetramethyl-piperidine and4-(4-hydroxycyclohexyl)-2,2,6,6-tetramethyl-piperidine, respectively,after transformation into an appropriate derivate (e.g. triflate), to anaryl-(cyclohexyl-)-aryl coupling reaction {Poetsch, Kontakte(Darmstadt), 1988 (2), p. 15)} with a moiety (X: ClMg—, BrMg—, IMg—,Li—, ClZn—, (HO)₂B—).

E.g. (Id) can be obtained in analogy to (Ic), in all the variants givenabove.

A further object of the invention is to provide compounds of formula(II) and a nematic liquid crystal mixture comprising at least onecompound of formula (I) or (II). Preferably the mixture comprises 0.05to 5% of one or several compounds of formula (I) and/or (II). Especiallyprefered are mixtures comprising 0.05 to 5% of one or several compoundsof formula (II).

wherein

-   R¹ is H or alkyl of 1 to 12 C atoms-   R² is H, a linear or branched alkyl group of 1 to 16 or linear or    branched alkenyl group of 2 to 16 C atoms, wherein in either case    one —CH₂— group can be replaced by —O—, it also being possible to    replace one or more H by F-   M¹ is —OC(═O)— or a single bond-   M³ is a single bond-   G¹-G² is —CH₂—CH— or —H═C—-   p is 0 or 1    is phenylen-1,4-diyl, optionally substituted by one or two F,    cyclohexylen-1,4-diyl, biphenyl-4,4′-diyl, optionally substituted by    one or two F per ring, 1,1′-cyclohexyl-phenyl-4,4′-diyl, the phenyl    moiety optionally substituted by one or two F, or    1,1′-phenylcyclohexyl-4,4′-diyl, the phenyl moiety optionally    substituted by one or two F, or 1,1′-bicyclohexyl-4,4′-diyl    is phenylen-1,4-diyl, optionally substituted by one or two F, or    cyclohexylen-1,4-diyl, with the proviso-   a) R² is H only in case p is 1-   b) if p is 0, than R² is a linear or branched alkyl group of 1 to 16    or linear or branched alkenyl group of 2 to 16 C atoms and    is cyclohexylen-1,4-diyl-   c) -G¹-G²- can be —CH═C— only in case M¹ is a single bond

A further object of the invention is to provide compounds of formula(III) and a chiral smectic liquid crystal mixture comprising at leastone compound of formula (III). Preferably the mixture comprises 0,05 to5% of one or several compounds of formula (III).

wherein

-   R¹ is H or alkyl of 1 to 12 C atoms-   R² is H, a linear or branched alkyl group of 1 to 16 or linear or    branched alkenyl group of 2 to 16 C atoms, wherein in either case    one —CH₂— group can be replaced by —O—, it also being possible to    replace one or more H by F-   M¹ is —OC(═O)— or a single bond-   M³ is a single bond-   G¹-G² is —CH₂—CH— or —CH═C—-   p is 0 or 1    is pyrimidin-2,5-diyl, pyridin-2,5-diyl, optionally ortho to N    substituted by F, phenylen-1,4-diyl, optionally substituted by one    or two F, or cyclohexylen-1,4-diyl, biphenyl-4,4′-diyl, optionally    substituted by one or two F per ring,    1,1′-cyclohexylphenyl-4,4′-diyl, the phenyl moiety optionally    substituted by one or two F, or 1,1′-phenylcyclohexyl-4,4′-diyl, the    phenyl moiety optionally substituted by one or two F, or    1,1′-bicyclohexyl-4,4′-diyl    is phenylen-1,4-diyl, optionally substituted by one or two F,    cyclohexylen-1,4-diyl, biphenyl-4,4′-diyl, optionally substituted by    one or two F per ring, 1,1′-cyclohexylphenyl-4,4′-diyl, the phenyl    moiety optionally substituted by one or two F, or    1,1′-phenylcyclohexyl-4,4′-diyl, the phenyl moiety optionally    substituted by one or two F, or 1,1′-bicyclohexyl-4,4′-diyl,    pyrimidin-2,5-diyl, or pyridin-2,5-diyl, optionally ortho to N    substituted by F,-   with the provisos-   a) one and only one of    or    is pyrimidin-2,5-diyl or pyridin-2,5-diyl, optionally ortho to N    substituted by F-   b) -G¹-G²- can be —CH═C— only in case M¹ is a single bond

Yet another object of the invention is to provide a liquid crystalmixture comprising at least 1 compound of formula (IV)

wherein R¹ to R⁶, A, M² and m have the meanings as in formula (I) and M¹is a single bond.

A further object of the invention is to provide a liquid crystal displaydevice, especially one operated in an active matrix panel using theabove described mixtures.

A further object of the invention is to provide a chiral smectic liquidcrystal display device, especially one operated in an active matrixpanel using the above described mixtures.

A further object of the invention is to provide a ferroelectric liquidcrystal display device, especially one operated in an active matrixpanel using the above described mixtures. With preference this displayis a monostable ferroelectric display, such as half-V-shape, CDR orshort pitch FLC displays.

A further object of the invention is to provide an antiferroelectricliquid crystal display device, especially one operated in an activematrix panel using the above described mixtures. With preference thisdisplay is a monostable antiferroelectric display, such as the so-called“V-shape” mode.

A further object of the invention is to provide a nematic liquid crystaldisplay device, especially one operated in an active matrix panel usingthe above described mixtures.

Yet a further object of the invention is the use of the above describedmixtures in a liquid crystal display, especially one operated in anactive matrix panel, especially if the liquid crystal is a chiralsmectic, particularly a monostable chiral smectic mode.

The liquid crystal mixtures according to the invention are prepared in amanner which is customary per se. As a rule the components are dissolvedin one another, advantageously at elevated temperatures.

The liquid crystal mixtures according to the invention generallycomprises of at least two (2), preferably at least five (5),particularly at least eight (8) compounds.

Reference is made to e.g. DE-A-1 985 7352 or DE-A 1 962 9812 (p. 12 to16) for the LC compounds that can be, besides the compounds of formula(I), (II), (III) or (IV), the constituents of mixtures according to theinvention.

Optional additional constituents of the mixtures according to inventionare materials that increase the light stability (UV stabilizers, e.g. ofthe “benzophenone” or “benzotriazole” type). Preferably the mixtures maycomprise 0.01 wt.-% to 10 wt.-% of one or several UV stabilizers;especially prefered are mixtures containing 0.1 wt.-% to 5 wt.-% of oneor several UV stabilizers.

Optional additional constitutents of the mixtures according to inventionare materials that increase the stability against oxidative degradation(antioxidants, e.g. of the “sterically hindered phenol” type).Preferably the mixtures may comprise 0.01 wt.-% to 10 wt.-% of one orseveral antioxidants; especially prefered are mixtures comprise 0.1wt.-% to 5 wt.-% of one or several antioxidants. Optionally the mixturesaccording to invention may comprise a combination of UV stabilizers andantioxidants.

The mixtures according to the invention can be used in electro-opticalor fully optical elements, for example display elements, switchingelements, light modulators, shutters, elements for image processingand/or signal processing or generally in the area of nonlinear optics.

Chiral smectic liquid crystal mixtures according to the invention areparticularly suitable for use in electro-optical switching and displaydevices (displays). These displays are usually constructed in such a waythat a liquid crystal layer is enclosed on both sides by layers whichare usually, in this sequence starting from the LC layer, at least onealignment layer, electrodes and a limiting sheet (for example of Dglass). In addition, they contain spacers, adhesive frames, polarizersand, for color displays, thin color-filter layers or are operated in thesequential backlight technique. Other possible components areantireflection, passivation, compensation and barrier layers and, foractive-matrix displays, electric non-linear elements, such as thin-filmtransistors (TFTS) and metal-insulator-metal (MIM) elements. Thestructure of liquid crystal displays has already been described indetail in relevant monographs (see, for example, T. Tsukuda, “TFT/LCDLiquid crystal displays addressed by thin fim transistors”, JapaneseTechnology Reviews, 1996 Gordon and Breach, ISBN 2-919875-01-91.

The present invention further provides a chiral smectic liquid crystal(FLC) display device comprising the above-mentioned liquid crystalmixture of the present invention between a pair of substrates eachcomprising an electrode and an alignment layer formed thereon.

In a preferred embodiment the FLC display is operated in the monostablemode with active matrix panel.

The present invention further provides a nematic liquid crystal displaydevice comprising the above-mentioned liquid crystal mixture of thepresent invention between a pair of substrates each comprising anelectrode and an alignment layer formed thereon. In a preferredembodiment the display is operated in the ECB mode (EP-A 0474062). IPSmode (Kiefer et al., Japan Display '92, S. 547) or VA mode (Ohmura etal., SID 97 Digest, S. 845).

Several documents are cited in this application, e.g. to discuss thestate of the art, synthesis of compounds used in the present inventionor application of the mixtures according to the invention. All thesedocuments are hereby incorporated by reference.

EXAMPLES Cell Fabrication

A solution of LQT 120 (Hitachi Kasei) is applied onto glass substrateswith ITO by spin coating at 2500 rpm. The substrates are heated at 200°C. for 1 hour to form a film. After rubbing the coated film with a nyloncloth in one direction, the substrates are assembled into a cell withspacers having a thickness of 2.0 μm inserted between the substrates insuch a manner that the rubbing directions are anti-parallel to eachother. The properties of the liquid crystal mixture are measured byfilling the liquid crystal mixture into the cell in the isotropic phase,cooling progressively through the nematic, (smectic A phase in case theliquid crystal mixture has one) and the smectic C phase and thenapplying a rectangular wave pulse (60 Hz) with varying applied voltage(0 to 10V) to the cell at 25° C. Resistivity and ion-induced spontaneouspolarization are measured by means of a MTR-1 device (Toya Technica).

Table 1 summarizes the results thus obtained by adding small quantitiesof several types of (I) to S_(c) mixture A and B, respectively. As canbe seen by comparing with the respective reference examples rA/rB{comprising no (I)}, the mixtures according to invention surprisinglyexhibit a higher resisitivity and decreased ion content. Hence themixtures according to invention can find application in active-matrixdevices.

TABLE 1 Mixture A Mixture B Ion Ion wt. Resistivity Cont. wt.Resistivity Cont. ex. % [TΩcm] [nC/cm²] % [TΩcm] [nC/cm²] rA/ Without(I) Trade Name 0 8.5 0.44 0 7.1 2.3 rB 1A 1B

Sanduvor 3052Liq. 0.1 >10 <0.1 0.1 7.22 0.24 2A 2B

Hostavin N20 (Sanduvor 3051 PDR) 0.1 >10 <0.1 0.1 >10 0.25 3A 3B

VP Sanduvor PR-31 0.1 >10 <0.1 0.1 3 0.24 4A 4B

TINUVIN□765 1.0 >10 <0.1 1.0 8.4 0.15 5A 5B

TINUVIN□770 1.0 >10 <0.1 1.0 8.2 0.18 6A with n = 13 to 16 LICOVIN 8450.1 >10 <0.1 0.1 8.0 <0.1 6B

Example 7

2,2,6,6-Tetramethyl-piperidine-4-(4-octyloxy)benzoate was obtained byesterifying 4-hydroxy-2,2,6,6-tetramethyl-piperidine with4-octyloxybenzoyl chloride in analogy to U.S. Pat. No. 4,038,280; m.p.79-81° C.

Example 8

2,2,6,6-Tetramethyl-piperidine-4-(4′-octylbiphenyl-4-yl)carboxylate) wasobtained in analogy to example 7 from 4′-octyl-biphenyl-4-yl carboxylicacid chloride; m.p. 104-106° C.

Example 9

2,2,6,6-Tetramethyl-piperidine-4-(trans-4-pentylcyclohexyl)carboxylatewas obtained in analogy to example 7 fromtrans-4-pentylcyclohexyl)carboxylic acid chloride; viscous liquid,crystallizes on standing (m.p. 44 to 48° C.).

Example 10

2,2,6,6-Tetramethyl-piperidine-4-[4-(trans-4-propylcyclohexyl)]benzoatewas obtained in analogy to example 7 from4-(trans-4-propylcyclohexyl)benzoyl chloride.

Example 11

2,2,6,6-Tetramethyl-piperidine-4-[4-(5-hexyl-pyrimidin-2-yl)]benzoatewas obtained in analogy to example 7 from4-(5-hexyl-pyrimidin-2-yl)benzoylchloride.

Example 12

2,2,6,6-Tetramethyl-piperidine-4-[4′-(4-decyloxy)benzoyloxy-biphenyl-4-yl]carboxylatewas obtained in analogy to example 7 from[4′-(4-decyloxy)benzoyloxy-biphenyl-4-yl]carboxylic acid chloride.

Example 13

2,2,6,6-Tetramethyl-piperidine-4-(4-hexyl)benzoate was obtained inanalogy to RO 92779 B1 (CAN 109:171562) by heating a xylene solution ofmethyl 4-hexylbenzoate, 4-hydroxy-2,2,6,6-tetramethylpiperidine andsodium methanolate to reflux for 10 hrs. Silica treatment andrecrystallization rendered pure material of m.p. 61-62° C.

Example 14

2,2,6,6-Tetramethylpiperidine-4-(4′-heptylbiphenyl-4-yl)carboxylate wasobtained in analogy to example 13 by subjecting methyl4′-heptylbiphenyl-4-carboxylate to the reaction with4-hydroxy-2,2,6,6-tetramethylpiperidine; m.p. 106-107° C.

Example 15

1,2,2,6,6-Pentamethyl-4-[trans-4-(4-propylcyclohexyl)phenyl]piperidinewas obtained in analogy to Skowronski et al., Pol. J. Chem. 54, 195,(1980) by reacting in tetrahydrofurane1,2,2,6,6-pentamethyl-piperidine-4-one with the Grignard reagentprepared from 4-(trans-4-propylcyclohexyl)bromobenzene, subjecting thecrude reaction product to azeotropic dehydration to3,4-dehydro-1,2,2,6,6-pentamethyl-4-[trans-4-(4-propylcyclohexyl)phenyl]piperidineand hydrogenation in tetrahydrofuran, catalyzed by Pd/C, at ambienttemperature and atmospheric pressure, followed by silica treatment, as aviscous liquid. ¹H-NMR (300 MHz, CDCl₃/DMSO/TMS) δ=7.18 m, 4H),2.65-2.49 (m.2H), 2.24 (s.3H), 1.95-1.80 (m,6H), 1.55-1.15 (m,9H),1.12-0.96 (m,14H), 0.91 (t, 3H)

Table 2 summarizes the results—(achieved with the equipment andprocedure of examples 1 to 6) obtained by adding small quantities ofseveral types of (I) to one of the following S_(c) mixtures:

-   M1 {achiral block mixture based on    5-alkyl-2-(4-alkyloxyphenyl)pyrimidines}-   M2 (chiral mixture based on derivatives of phenylpyrimidines)-   M3b (achiral multi-component mixture based on ester and ether    derivatives of phenylpyrimidines, fluorinated bi- and terphenyls and    sulfur heterocycles)-   M3 (like M³b, but chiral dopant added)

TABLE 2 Ion Content [nc/cm²] storage storage storage storage storageexample condition mixture 0 hrs 100 hrs 200 hrs 300 hrs 500 hrsreference.  20 V, 5 Hz M1 <0.01 0.3 0.35 0.48 0.25 16  20 V, 5 Hz M1 +5% <0.01 <0.01 <0.01 <0.01 <0.01 expl. 13 reference.  20 V, 5 Hz M3b0.21 4.69 4.36 4.37 4.32 17  20 V, 5 Hz M3b + 0.1% 0.17 0.3 0.48 0.470.54 expl. 13 18  20 V, 5 Hz M3b + 0.2% 0.08 0.23 0.23 0.25 0.27 expl.13 19  20 V, 5 Hz M3b + 0.4% 0.16 0.23 0.2 0.22 0.25 expl. 13 reference.5 V, 0.1 Hz M4 0 2.56 7.57 3.48 6.33 20 5 V, 0.1 Hz M4 + 0.4% <0.01<0.01 0.12 0.22 0.17 expl. 13 reference. 5 V, 0.1 Hz M2 0.2 8.18 10.939.49 6.43 21 5 V, 0.1 Hz M2 + 0.4% <0.01 <0.01 0.08 0.48 0.73 expl. 1322 5 V, 0.1 Hz M2 + 0.2% <0.01 1.76 2.37 2.08 1.87 expl. 14 23 5 V, 0.1Hz M2 + 0.4% <0.01 1.14 1.57 1.41 no data expl. 14 24 5 V, 0.1 Hz M4 +0.2% <0.01 1.91 2.71 2.79 2.94 expl. 14 25 5 V, 0.1 Hz M4 + 0.4% <0.012.02 2.80 3.09 2.91 expl. 14

As can be seen by comparing with the respective reference examples{comprising no (I)}, the mixtures according to invention surprisinglyexhibit a substantially lower ion content [nC/cm²] over a longer periodof time.

1. Chiral smectic liquid crystal mixture comprising one or severalcompounds of formula (I) wherein

R¹: is H or a linear or branched alkyl group of 1 to 20 or a linear orbranched alkenyl group of 2 to 20 C atoms, wherein in either caseoptionally one —CH₂— group being replaced by cyclohexylen-1,4-diyl, orone or two —CH₂— groups optionally being replaced by —O—, ifnon-adjacent to N, or by —C(═O)—, —Si(CH₃)₂—, and/or one or more H ofthe alkyl or alkenyl group optionally being replaced by F or CH₃; R²:stands for a) H or F, b) a linear or branched alkyl group of 1 to 20 ora linear or branched alkenyl group of 2 to 20 C atoms, wherein in eithercase one or two —CH₂— groups optionally being replaced by —O—, —C(═O)O—,—Si(CH₃)₂—, and/or one or more H of the alkyl or alkenyl group beingreplaced by F or CH₃, c) a radical

wherein independently from the respective meanings in (I) R³, R⁴, R⁵,R⁶: are independently from another an alkyl group of 1 to 8 C atoms; M¹,M²: represent independently from another a single bond, —OC(═O)—,—C(═O)O—, —OCH₂—, —NH—; A: is a) a linear or branched alkan-α,ω-diylgroup of 1 to 20 or alkene-α,ω-diyl group of 2 to 20 C atoms,optionally, if non-adjacent to M¹ and M², one or two non-adjacent —CH₂—groups may be replaced by —O—, b) the group —C(═Y)— wherein Y is CH-Zwith Z being phenylen-1,4-diyl, optionally substituted by one to threehalogen atoms, alkyl or alkyloxy groups of 1 to 4 C atoms, with theproviso that M¹ and M² are —C(═O)O— and —OC(═O)—, c) the group —CHYwherein Y is CH₂-Z with Z being phenylen-1,4-diyl, optionallysubstituted by one to three halogen atoms, alkyl or alkyloxy groups of 1to 4 C atoms, with the proviso that M¹ and M² are —C(═O)O— and —OC(═O)—,d) a group

wherein p, q are 1 or 2, the sum of p+q being ≧1; M³ is a single bond or—OC(═O)—, —C(═O)O—, —OCH₂—, —CH₂O—, —C≡C—, —CH₂CH₂—, —CH₂CH₂CH₂CH₂—, theradicals

are independently from another phenylen-1,4-diyl, which optionally canbe substituted by one, two or three F, or cyclohexan-1,4-diyl, which canoptionally be substituted by one CN, CH₃ or F, or pyrimidin-2,5-diyl,optionally substituted by one F, pyridine-2,5-diyl, which can optionallybe substituted by one F, or naphthalene-2,6-diyl, which can optionallybe substituted by one, two or three F, or1,2,3,4-tetrahydronaphthalene-2,6-diyl (the aromatic ring optionallysubstituted by one, two or three F), or decahydronaphthalene-2,6-diyl,or indane-2,5(6)-diyl, or fluorene-2,-7-diyl, or phenanthrene-2,7-diyl,or 9,10-dihydrophenanthrene-2,7-diyl, or (1,3,4)thiadiazol-2,5-diyl, or(1,3)thiazol-2,5-diyl, or (1,3)thiazol-2,4-diyl, or thiophen-2,4-diyl,or thiophen-2,5-diyl, or (1,3)dioxan-2,5-diyl, or piperidin-1,4-diyl, orpiperazin-1,4-diyl; X is H, OH, a linear or branched alkyl or alkyloxygroup of 1 to 20 C atoms, wherein one or two —CH₂— groups can bereplaced by —O—, —C(═O)O—, —Si(CH₃)₂—, optionally one or more H beingreplaced by F or CH₃; m: is 0 or 1; X and M¹-(A)_(m)-M²-R² together canconstitute a) a ring of 4 to 16 members, optionally substituted by analkyl radical of 1 to 15 C atoms, b) a combination of two eitherdirectly linked or spiro-linked rings of independently from another 4 to16 members, optionally substituted by an alkyl radical of 1 to 15 Catoms in either of the three variants the rings independently fromanother being carbocycles or carbocycles comprising B, N, O or Sheteroatoms.
 2. Chiral smectic liquid crystal mixture according to claim1 wherein the mixture comprises 0.01 wt.-% to 10 wt.-% of one or severalcompounds of formula (I).
 3. Chiral smectic liquid crystal mixtureaccording to claim 1, comprising at least one compound of the followingformulae

wherein, n is an integer from 1 to 20; X is H; R¹ is H or alkyl of 1 to4 C atoms; R³, R⁴, R⁵, R⁶ are CH₃; and/or

wherein X is H; R¹ as H or alkyl of 1 to 4 C atoms; R³, R⁴, R⁶, R⁵ areCH₃; and/or

wherein X is H; R¹ is H or alkyl of 1 to 4 C atoms; R³, R⁴, R⁵, R⁶ areCH₃; and/or

wherein X is H; R¹ is H or alkyl of 1 to 4 C atoms; R³, R⁴, R⁵, R⁶ areCH₃.
 4. Chiral smectic liquid crystal mixture according to claim 1comprising at least one compound of the following formulae

wherein X is H; R¹ is H or alkyl of 1 to 4 C atoms; R³, R⁴, R⁵, R⁶ areCH₃; M¹ is —OC(═O)—; M² is —C(═O)O—; n is 4 to 12; and/or

wherein X is H; R¹ is H or alkyl of 1 to 4 C atoms; R³, R⁴, R⁵, R⁶ areCH₃; R² is H; M¹ is —OC(═O)—; M² is a single bond; n is 8 to 20; and/or

wherein X is H; R¹ is H or alkyl of 1 to 4 C atoms; R³, R⁴, R⁵, R⁶ areCH₃; R² is H or an alkyl or alkyloxy group of 1 to 16 C atoms, whereinone or two —CH₂— groups can be replaced by —O—, —OC(═O)—, —Si(CH₃)₂—, italso being possible to replace one or more H by F or CH₃; M¹ is—OC(═O)—; M² is a single bond;

is phenylen-1,4-diyl, optionally substituted by one or two F, orcyclohexylen-1,4-diyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl; and/or

wherein X is H; R¹ is H or alkyl of 1 to 4 C atoms; R³, R⁴, R⁵, R⁶ areCH₃; R² is H or an alkyl or alkyloxy group of 1 to 16 C atoms, whereinone or two —CH₂— groups can be replaced by —O—, —OC(═O)—, —Si(CH₃)₂—, italso being possible to replace one or more H by F or CH₃; M¹ is—OC(═O)—; M² is a single bond; M³ is a single bond, —OC(═O)—, —OCH₂—;

is phenylen-1,4-diyl, optionally substituted by one or two F, orcyclohexylen-1,4-diyl, pyrimidine-2,5-diyl, or pyridine-2,5-diyl;

is phenylen-1,4-diyl, optionally substituted by one or two F, orcyclohexylen-1,4-diyl, pyrimidine-2,5-diyl, or pyridine-2,5-diyl. 5.Chiral smectic liquid crystal mixture comprising one or severalcompounds of formula (IV)

wherein R¹: is H, or a linear or branched alkyl group of 1 to 20 or alinear or branched alkenyl group of 2 to 20 C atoms, wherein in eithercase optionally one —CH₂— group being replaced by cyclohexylen-1,4-diyl,or one or two —CH₂— groups optionally being replaced by —O—, ifnon-adjacent to N, or by —C(═O)—, —Si(CH₃)₂—, and/or one or more H ofthe alkyl or alkenyl group optionally being replaced by F or CH₃; R²:stands for a) H or F, b) a linear or branched alkyl group of 1 to 20 ora linear or branched alkenyl group of 2 to 20 C atoms, wherein in eithercase one or two —CH₂— groups optionally being replaced by —O—, —C(═O)O—,—Si(CH₃)₂—, and/or one or more H of the alkyl or alkenyl group beingreplaced by F or CH₃, c) a radical

wherein independently from the respective meanings in (I) R³, R⁴, R⁵,R⁶: are independently from another an alkyl group of 1 to 8 C atoms; M¹,M²: represent independently from another a single bond, —OC(═O)—,—C(═O)O—, —OCH₂—, —NH—; A: is a) a linear or branched alkan-α,ω-diylgroup of 1 to 20 or alkene-α,ω-diyl group of 2 to 20 C atoms,optionally, if non-adjacent to M¹ and M², one or two non-adjacent —CH₂—groups may be replaced by —O—, b) the group —C(═Y)— wherein Y is CH-Zwith Z being phenylen-1,4-diyl, optionally substituted by one to threehalogen atoms, alkyl or alkyloxy groups of 1 to 4 C atoms, with theproviso that M¹ and M² are —C(═O)O— and —OC(═O)—, c) the group —CHYwherein Y is CH₂-Z with Z being phenylen-1,4-diyl, optionallysubstituted by one to three halogen atoms, alkyl or alkyloxy groups of 1to 4 C atoms, with the proviso that M¹ and M² are —C(═O)O— and —OC(═O)—,d) a group

wherein p, q are 1 or 2, the sum of p+q being ≧1; M³ is a single bond or—OC(═O)—, —C(═O)O—, —OCH₂—, —CH₂O—, —C≡C—, —CH₂CH₂—, —CH₂CH₂CH₂CH₂—; theradicals

are independently from another phenylen-1,4-diyl, which optionally canbe substituted by one, two or three F, or cyclohexan-1,4-diyl, which canoptionally be substituted by one CN, CH₃ or F, or pyrimidin-2,5-diyl,optionally substituted by one F, pyridine-2,5-diyl, which can optionallybe substituted by one F, or naphthalene-2,8-diyl, which can optionallybe substituted by one, two or three F, or1,2,3,4-tetrahydronaphthalene-2,6-diyl (the aromatic ring optionallysubstituted by one, two or three F), or decahydronaphthalene-2,6-diyl,or indane-2,5(6)-diyl, or fluorene-2,-7-diyl, or phenanthrene-2,7-diyl,or 9,10-dihydrophenanthrene-2,7-diyl, or (1,3,4)thiadiazol-2,5-diyl, or(1,3)thiazol-2,5-diyl, or (1,3)thiazol-2,4-diyl, or thiophen-2,4-diyl,or thiophen-2,5-diyl, or (1,3)dioxan-2,5-diyl, or piperidin-1,4-diyl, orpiperazin-1,4-diyl; X is H, OH, a linear or branched alkyl or alkyloxygroup of 1 to 20 C atoms, wherein one or two —CH₂— groups can bereplaced by —O—, —C(═O)O—, —Si(CH₃)₂—, optionally one or more H beingreplaced by F or CH₃; m: is 0 or 1; and M¹ is a single bond.
 6. Chiralsmectic liquid crystal mixture comprising at least two compounds offormula (I) and/or (IV) wherein

R¹: is H or a linear or branched alkyl group of 1 to 20 or a linear orbranched alkenyl group of 2 to 20 C atoms, wherein in either caseoptionally one —CH₂— group being replaced by cyolohexylen-1,4-diyl, orone or two —CH₂— groups optionally being replaced by —O—, ifnon-adjacent to N, or by —C(═O)—, —Si(CH₃)₂—, and/or one or more H ofthe alkyl or alkenyl group optionally being replaced by F or CH₃; R²:stands for a) H or F, b) a linear or branched alkyl group of 1 to 20 ora linear or branched alkenyl group of 2 to 20 C atoms, wherein in eithercase one or two —CH₂— groups optionally being replaced by —O—, —C(═O)O—,—Si(CH₃)₂—, and/or one or more H of the alkyl or alkenyl group beingreplaced by F or CH₃, c) a radical

wherein independently from the respective meanings in (I) R³, R⁴, R⁵,R⁶: are independently from another an alkyl group of 1 to 8 C atoms; M¹,M²: represent independently from another a single bond, —OC(═O)—,—C(═O)O—, —OCH₂—, —NH—; A: is a) a linear or branched alkan-α,ω-diylgroup of 1 to 20 or alkene-α,ω-diyl group of 2 to 20 C atoms,optionally, if non-adjacent to M¹ and M², one or two non-adjacent —CH₂—groups may be replaced by —O—, b) the group —C(═Y)— wherein Y is CH-Zwith Z being phenylen-1,4-diyl, optionally substituted by one to threehalogen atoms, alkyl or alkyloxy groups of 1 to 4 C atoms, with theproviso that M¹ and M² are —C(═O)O— and —OC(═O)—, c) the group —CHYwherein Y is CH₂-Z with Z being phenylen-1,4-diyl, optionallysubstituted by one to three halogen atoms, alkyl or alkyloxy groups of 1to 4 C atoms, with the proviso that M¹ and M² are —C(═O)O— and —OC(═O)—,d) a group

wherein p, q are 1 or 2, the sum of p+q being ≧1; M³ is a single bond or—OC(═O)—, —C(═O)O—, —OCH_(2—, —CH) ₂O—, —C≡C—, —CH₂CH₂—, —CH₂CH₂CH₂CH₂—;the radicals

are independently from another phenylen-1,4-diyl, which optionally canbe substituted by one, two or three F, or cyclohexan-1,4-diyl, which canoptionally be substituted by one CN, CH₃ or F, or pyrimidin-2,5-diyl,optionally substituted by one F, pyridine-2,5-diyl, which can optionallybe substituted by one F, or naphthalene-2,6-diyl, which can optionallybe substituted by one, two or three F, or1,2,3,4-tetrahydronaphthalene-2,6-diyl (the aromatic ring optionallysubstituted by one, two or three F), or decahydronaphthalene-2,6-diyl,or indane-2,5(6)-diyl, or fluorene-2,-7-diyl, or phenanthrene-2,7-diyl,or 9,10-dihydrophenanthrene-2,7-diyl, or (1,3,4)thiadiazol-2,5-diyl, or(1,3)thiazol-2,5-diyl, or (1,3)thiazol-2,4-diyl, or thiophen-2,4-diyl,or thiophen-2,5-diyl, or (1,3)dioxan-2,5-diyl, or piperidin-1,4-diyl, orpiperazin-1,4-diyl; X is H, OH, a linear or branched alkyl or alkyloxygroup of 1 to 20 C atoms, wherein one or two —CH₂— groups can bereplaced by —O—, —C(═O)O—, —Si(CH₃)₂—, optionally one or more H beingreplaced by F or CH₃; m: is 0 or 1; X and M¹-(A)_(m)-M²-R² together canconstitute a) a ring of 4 to 16 members, optionally substituted by analkyl radical of 1 to 15 C atoms, b) a combination of two eitherdirectly linked or spiro-linked rings of independently from another 4 to16 members, optionally substituted by an alkyl radical of 1 to 15 Catoms in either of the three variants the rings independently fromanother being carbocycles or carbocycles comprising B, N, O or Sheteroatoms; and wherein

wherein R¹: is H or a linear or branched alkyl group of 1 to 20 or alinear or branched alkenyl group of 2 to 20 C atoms, wherein in eithercase optionally one —CH₂— group being replaced by cyclohexylen-1,4-diyl,or one or two —CH₂— groups optionally being replaced by —O—, ifnon-adjacent to N, or by —C(═O)—, —Si(CH₃)₂—, and/or one or more H ofthe alkyl or alkenyl group optionally being replaced by F or CH₃; R²:stands for a) H or F, b) a linear or branched alkyl group of 1 to 20 ora linear or branched alkenyl group of 2 to 20 C atoms, wherein in eithercase one or two —CH₂— groups optionally being replaced by —O—, —C(═O)O—,—Si(CH₃)₂—, and/or one or more H of the alkyl or alkenyl group beingreplaced by F or CH, c) a radical

wherein independently from the respective meanings in (I) R³, R⁴, R⁵,R⁶: are independently from another an alkyl group of 1 to 8 C atoms; M¹,M²: represent independently from another a single bond, —OC(═O)—,—C(═O)O—, —OCH₂—, —NH—; A: is a) a linear or branched alkan-α,ω-diylgroup of 1 to 20 or alkene-α,ω-diyl group of 2 to 20 C atoms,optionally, if non-adjacent to M¹ and M², one or two non-adjacent —CH₂—groups may be replaced by —O—, b) the group —C(═Y)— wherein Y is CH-Zwith Z being phenylen-1,4-diyl, optionally substituted by one to threehalogen atoms, alkyl or alkyloxy groups of 1 to 4 C atoms, with theproviso that M¹ and M² are —C(═O)O— and —OC(═O)—, c) the group —CHYwherein Y is CH₂-Z with Z being phenylen-1,4-diyl, optionallysubstituted by one to three halogen atoms, alkyl or alkyloxy groups of 1to 4 C atoms, with the proviso that M¹ and M² are —C(═O)O— and —OC(═O)—,d) a group

wherein p, q are 1 or 2, the sum of p+q being ≧1; M³ is a single bond or—OC(═O), —C(═O)O—, —OCH₂—, —CH₂O—, —C≡C—, —CH₂CH₂—, —CH₂CH₂CH₂CH₂—; theradicals

are independently from another phenylen-1,4-diyl, which optionally canbe substituted by one, two or three F, or cyclohexan-1,4-diyl, which canoptionally be substituted by one CN, CH₃ or F, or pyrimidin-2,5-diyl,optionally substituted by one F, pyridine-2,5-diyl, which can optionallybe substituted by one F, or naphthalene-2,6-diyl, which can optionallybe substituted by one, two or three F, or1,2,3,4-tetrahydronaphthalene-2,6-diyl (the aromatic ring optionallysubstituted by one, two or three F), or decahydronaphthalene-2,6-diyl,or indane-2,5(6)-diyl, or fluorene-2,-7-diyl, or phenanthrene-2,7-diyl,or 9,10-dihydrophenanthrene-2,7-diyl, or (1,3,4)thiadiazol-2,5-diyl, or(1,3)thiazol-2,5-diyl, or (1,3)thiazol-2,4-diyl, or thiophen-2,4-diyl,or thiophen-2,5-diyl, or (1,3)dioxan-2,5-diyl, or piperidin-1,4-diyl, orpiperazin-1,4-diyl; X is H, OH, a linear or branched alkyl or alkyloxygroup of 1 to 20 C atoms, wherein one or two —CH₂— groups can bereplaced by —O—, —C(═O)O—, —Si(CH₃)₂—, optionally one or more H beingreplaced by F or CH₃; m: is 0 or 1; and M¹ is a single bond.
 7. Chiralsmectic liquid crystal mixture according to claim 1, wherein the mixturefurther comprises one or several antioxidants.
 8. Chiral smectic liquidcrystal mixture according to claim 7, wherein the mixture comprises 0.01to 10 wt-% of one or several antioxidants.
 9. Chiral smectic liquidcrystal mixture according to claim 7, wherein the mixture furthercomprises one or several UV-stabilizers.
 10. Liquid crystal displaydevice comprising a chiral liquid crystal mixture according to claim 1.11. Liquid crystal display device according to claim 10, wherein thedevice is operated in an active-matrix panel.
 12. Liquid crystal displaydevice according to claim 10, wherein the display is a monostabledisplay.
 13. Compounds of the formula (II)

wherein R¹ is H or alkyl of 1 to 12 C atoms; R² is H a linear orbranched alkyl group of 1 to 16 or linear or branched alkenyl group of 2to 16 C atoms, wherein in either case one —CH₂— group can be replaced by—O—, it also being possible to replace one or more H by F; M¹ is—OC(═O)— or a single bond; M³ is a single bond; G¹-G² is —CH₂—CH— or—CH═C—, p is 0 or 1;

is phenylen-1,4-diyl, optionally substituted by one or two F,cyclohexylen-1,4-diyl, biphenyl-4,4′-diyl, optionally substituted by oneor two F per ring, 1,1′-cyclohexyl-phenyl-4,4′-diyl, the phenyl moietyoptionally substituted by one or two F, or1,1′-phenylcyclohexyl-4,4′-diyl, the phenyl moiety optionallysubstituted by one or two F, or 1,1′-bicyclohexyl-4,4′-diyl;

is phenylen-1,4-diyl, optionally substituted by one or two F, orcyolohexylen-1,4-diyl, with the provisos a) R² is H only in case p is 1,b) if p is 0, than R² is a linear or branched alkyl group of 1 to 16 orlinear or branched alkenyl group of 2 to 16 C atoms and

is cyclohexylen-1,4-diyl, c) -G¹-G²- can be —CH═C— only in case M¹ is asingle bond.
 14. Compounds of the formula (III)

wherein R¹ is H or alkyl of 1 to 12 C atoms: R² is H, a linear orbranched alkyl group of 1 to 16 or linear or branched alkenyl group of 2to 16 C atoms, wherein in either case one —CH₂— group can be replaced by—O—, it also being possible to replace one or more H by F; M¹ is—OC(═O)— or a single bond; M³ is a single bond; G¹-G² is —CH₂—CH— or—CH═C—, p is 0 or 1;

is pyrimidin-2,5-diyl, pyridin-2,5-diyl, optionally ortho to Nsubstituted by F, phenylen-1,4-diyl, optionally substituted by one ortwo F, or cyclohexylen-1,4-diyl, biphenyl-4,4′-diyl, optionallysubstituted by one or two F per ring, 1,1′-cyclohexylphenyl-4,4′-diyl,the phenyl moiety optionally substituted by one or two F, or1,1′-phenylcyclohexyl-4,4′diyl, the phenyl moiety optionally substitutedby one or two F, or 1,1′-bicyclohexyl-4,4′diyl;

is phenylen-1,4-diyl, optionally substituted by one or two F,cyclohexylen-1,4-diyl, biphenyl-4,4′-diyl, optionally substituted by oneor two F per ring, 1,1′-cyclohexylphenyl-4,4′-diyl, the phenyl moietyoptionally substituted by one or two F, or1,1′-phenylcyclohexyl-4,4′-diyl the phenyl moiety optionally substitutedby one or two F, or 1,1′-bicyclohexyl-4,4′-diyl, pyrimidin-2,5-diyl, orpyridin-2,5-diyl, optionally ortho to N substituted by F, with theprovisos a) one and only one of

is pyrimidin-2,5-diyl or pyridin-2,5-diyl, optionally ortho to Nsubstituted by F, b) -G¹-G²- can be —CH═C— only in case M¹ is a singlebond.